Storm Clouds May Punch Holes in Ozone

Below:

Next story in Science

The same process that creates an ozone hole over Antarctica shows
up above big summer storm clouds in the United States and could
be destroying ozone there, a new study proposes.

Ninety percent of Earth's ozone is in the stratosphere (the
second layer of the atmosphere, just above the one we breathe,
the troposphere). This ozone forms the
ozone layer, which protects everything on the Earth's surface
from the sun's harmful ultraviolet rays.

Ozone destruction in the lower stratosphere is caused by
reactive chlorine and bromine molecules called free radicals. The
free radicals steal one of ozone's three oxygen atoms. Losing an
atom transforms ozone into an everyday oxygen molecule that
doesn't confer the same protective benefits.

Usually, the chemical reactions that zap ozone happen only at
extremely cold temperatures — about minus 112 degrees Fahrenheit
(minus 80 degrees Celsius) — such as high in the atmosphere above
the frozen poles. But Jim Anderson, lead author of the new study,
suspected increasing the concentration of water vapor in the
stratosphere
through storm clouds meant the chemical reactions would spark
at higher temperatures seen above the mid-latitudes of the United
States.

"Anytime you satisfy the proper concentration of water and
temperature, these reactions will take place," said Anderson, an
atmospheric chemistry professor at Harvard University.

Towering cumulonimbus clouds, the culprit behind severe storms,
carry water vapor high into the sky via convection. During
monitoring flights conducted between 2001 and 2007, scientists
found the cloud tops reached into the lower stratosphere, at
altitudes between 9 and 12 miles (15 and 20 kilometers) above us,
sending the water they contained aloft. [ Infographic:
Layers of Earth's Atmosphere ]

"These convective storms inject water far more deeply into the
stratosphere than anybody ever suspected, and that immediately
began to concern us," Anderson told OurAmazingPlanet.

Anderson's key concern is chlorine free radicals. The atoms
assume their dangerous nature during a rapid series of chemical
reactions that take place on the surface of sulfate-water
aerosols, tiny droplets that are ubiquitous in the lower
stratosphere.

"The chemistry of these sulfate-water aerosols is such that
conversion rate from inorganic chlorine is a function not just of
temperature, but of water vapor and temperature," Anderson said.
Adding more water vapor to the lower stratosphere means chlorine
free radicals can form at higher temperatures, he explained. The
abundant sunlight available at lower latitudes also boosts the
reaction speed.

The study shows chlorine molecules build up rapidly, within the
first 24 hours after a storm. As a result, ozone loss can
increase by two orders of magnitude compared to that in the
regularly arid stratosphere.

"We were quite surprised by the rate that the chemistry responds
to the sunlight conditions in the lower stratosphere," Anderson
said. "This conversion takes place in one diurnal period, and
that's a lot more rapidly than we had expected."

Direct observations needed

Computer modeling, combined with data from research aircraft,
indicates the right conditions occur above the central United
States during summertime, the new study finds.

However, no one has directly observed the process taking place.
That makes other scientists intrigued but cautious about the
theory.

"I don't think Jim's proven the case yet," said Andrew Dessler,
professor of atmospheric chemistry at Texas A&M University.
"We have no data to refute or support the idea."

Mary Barth, a scientist with the National Center for Atmospheric
Research, said Anderson's paper will encourage the research
community to "look at things a little differently."

"Understanding how much stuff gets into the stratosphere is
something we're still figuring out in detail. What he is doing is
pushing us to really evaluate his theory and get some data to see
if it makes sense," she said.

Anderson and his co-authors call on NASA to fly research aircraft
through stratospheric cloud tops to confirm evidence of ozone
hole chemistry in their paper, published today (July 26) in
Science Express, the online edition of the journal Science

"We have a very powerful foundation of chemistry that we have
verified through many observations of a wide range of
conditions," Anderson said. "It's true that if we had our way, we
would like to observe this within a convective event over the
United States."Future research could also show whether the
process is confined to storms, or if water vapor circulates in
the stratosphere, allowing free radicals to deplete ozone over a
wide area.

"If the air blows downwind and it's gone, then this is a
curiosity," Dessler said. "On the other hand, there might be
enough water vapor coming out of these events to have a
detectable effect on ozone. If it's happening, we want to know if
this is important on a hemispheric scale or does this have an
effect on global ozone."